Body-centered-cubic martensite and the role on room-temperature tensile properties in Si-added SiVCrMnFeCo high-entropy alloys

We present a new class of metastable high-entropy alloys (HEAs), triggering deformation-induced martensitic transformation (DIMT) from face-centered-cubic (FCC) to body-centered-cubic (BCC), i.e., BCC-DIMT. Through the ab-initio calculation based on 1^st order axial interaction model and combined with the Gibbs free energy calculation, the addition of Si is considered as a critical element which enables to reduce the intrinsic stacking fault energy (ISFE) in Si_xV_(9-x)Cr₁₀Mn₅Fe₄₆Co₃₀ (x = 2, 4, and 7 at.%) alloy system. The ISFE decreases from -30.4 to -35.5 mJ/m² as the Si content increases from 2 to 7 at.%, which well corresponds to the reduced phase stability of FCC against HCP. The BCC-DIMT occurs in all the alloys via intermediate HCP martensite, and the HCP martensite provides nucleation sites of BCC martensite. Therefore, the transformation rate enhances as the Si content increases in an earlier deformation range. However, the BCC-DIMT is also affected by the phase stability of FCC against BCC, and the stability is the highest at the Si content of 7 at.%. Thus, the 7Si alloy presents the moderate transformation rate in the later deformation range. Due to the well-controlled transformation rate and consequent strain-hardening rate, the 7Si alloy possesses the superior combination of strength and ductility beyond 1 GPa of tensile strength at room temperature. Our results suggest that the Si addition can be a favorable candidate in various metastable HEAs for the further property improvement.